OK, M&S is very erratic so disregard that!
The cup volume of a 38B is equal to a 36C, 34D and so on. Your 38B feels too tight because the cups are too small and your breasts are having to 'steal' room from the band and M&S bands are very random.
If you scoop whilst wearing your 38B, do you get overspill, under arm bulge, back fat?